Non-volatile memories are semiconductor memories that retain their contents when unpowered. Non-volatile memories are used for storage in electronic devices such as digital cameras, cellular telephones, music players, as well as in general computer systems, embedded systems and other electronic devices that require persistent storage. Non-volatile memories may be in the form of removable and portable memory cards or other memory modules. Non-volatile memories may be integrated into other types of circuits or devices, or can take any other desired form. Non-volatile memories are becoming more prevalent due to their small size, persistence, lack of moving parts, and reduced power requirements.
Flash memory is a common type of non-volatile memory used in a variety of devices. Flash memory uses an architecture that is proving inadequate in its access, erase and write times for the rapidly increasing operational speed requirements of electronic devices. Flash memory is further limited by its cost per unit storage and limited in the erasure of data in blocks of data rather than single bytes like random access memory (RAM). Memory cells of flash memory have a limited lifespan, such as around 100,000 write cycles, thus making it an inappropriate alternative to RAM.
Volatile memories, such as RAM can potentially be replaced by non-volatile memories. A non-volatile memory may replace RAM if the speed of non-volatile memory is increased to meet the requirements for RAM and other applications currently using volatile memories. One type of non-volatile memory may be variable resistance memory and may provide alternative to flash memory and random access memory (RAM) in a number of applications.
Variable resistance memory includes a variable resistance material (e.g. a dielectric material such as a metal oxide) that may change from a first resistivity to a second resistivity upon the application of a set voltage, and from the second resistivity back to the first resistivity upon the application of a reset voltage. Variable resistance memories use set and reset voltages that have opposite polarities relative to a common electrical reference (a common electrical reference may include a ground). When variable resistance memory elements are incorporated into a memory array, a select device is included to facilitate the selection of a desired memory bit. With existing variable resistance memories, diodes have been employed for selection of the desired memory bit.
The use of diodes as select devices for selection of a desired memory bit in variable resistance memory array is limited by a property of the diode to only conduct current in a single direction under one bias polarity. Additionally, the use of diodes for selection of the desired memory bit requires significant space which limits the ability to reduce the size of variable resistance memories. Significant space is required for fabrication of a variable resistance memory since diodes are not stackable upon the variable resistance material during fabrication of the variable resistance memory. Thus, what is needed is a variable resistance memory including a select device which provides bidirectional switching and may be stacked with layers of the variable resistance memory array to reduce the size of variable resistance memory.